The deepwater oxygen deficit in stratified shallow seas is mediated by diapycnal mixing

Seasonally stratified shelf seas are amongst the most biologically productive on the planet. A consequence is that the deeper waters can become oxygen deficient in late summer. Predictions suggest global warming will accelerate this deficiency. Here we integrate turbulence timeseries with vertical profiles of water column properties from a seasonal stratified shelf sea to estimate oxygen and biogeochemical fluxes. The profiles reveal a significant subsurface chlorophyll maximum and associated mid-water oxygen maximum. We show that the oxygen maximum supports both upward and downwards O2 fluxes. The upward flux is into the surface mixed layer, whilst the downward flux into the deep water will partially off-set the seasonal O2 deficit. The results indicate the fluxes are sensitive to both the water column structure and mixing rates implying the development of the seasonal O2 deficit is mediated by diapcynal mixing. Analysis of current shear indicate that the downward flux is supported by tidal mixing, whilst the upwards flux is dominated by wind driven near-inertial shear. Summer storminess therefore plays an important role in the development of the seasonal deep water O2 deficit.

74-75: Here the leakage as a cause for the O2 deficit is clearly formulated.Would be important to also have such a clear formulafion in the abstract.95-112: I find this paragraph very hard to read, because it reports several results from dissipafion obervafions at different fime and different depths.This feels very descripfive.The authors should find a way to reformulate this paragraph, to give a clear message.
110-112: "… This data quality issues …", not clear what these "issues" are, since observing dissipafion rate inside the SML should not be an issue.
115-116: Maybe befter "an average downward diapycnal heat flux of 40 W/m2"?138-140: Wouldn't it be possible to esfimate the oxygen flux to the atmosphere using bulk formulae as they are used in numerical models, depending on the wind speed and (maybe even the waves state)?146: "there" instead of "There".

Figs. 3c&h:
Here the symbols should get a legend within the panel, for befter readability.
155-158: I guess these are the values for June, but you did not write it explicitly.
155-176: Although plausible, these numbers are not easy to digest.Would it be possible to addifionally indicate these fluxes graphically, by some box plots with verfical arrows with size proporfional to the fluxes?I know that space is limited in this journal, but such a graphics would help the reader tremendiously.Maybe, table 1 (which could be much befter organised) could be replaced by such a graphics.
190: Why do you define N2/S2 = 1 as the value of marginal stability?Theory says that it should be 0.25.When you decrease N2/S2 from 1.0 to 0.9, the water column should sfill be stably strafified.This seems to contradict what you write ("any shear enhancement will potenfially generate shear instability and mixing").
Fig. 4a: It would be more consistent to plot that wind contribufion as wind speed fimes wind stress.It would also be befter to apply a 25-hour filter on the U3 values.Then, in both cases, the pink and blue areas would be more plausible.

226-230:
Here the argumentafion could be more clear.I understand that the decay of oxygen in the deep water due to usage for mineralisafion is larger than the supply from diapycnal mixing.Therefore, the oxygen concentrafion int he deep water is going down.But in what sense does this acclerate the removal?Are you saying that we have a self-enhancing process here?How does that work?

Reviewer #2 (Remarks to the Author):
This is an excellent paper.Tt's very clear and straighfforward despite the fact that it summarizes an extensive and diverse interdisciplinary data set.The main results are the clear demonstrafion that the mixing of DO downward from the pycnocline delays the decline in DO in the deepwater caused by respirafion, and than the mixing flux is driven by interior shear driven by fides and augmented by near inerfial oscillafions.This paper should be read by all coastal marine scienfists clarify the idea that low DO is due to reduced mixing because of summer increases in strafificafion.
The paper provides good evidence to support the claims and the methodology is sound.
I have only two suggesfions: in the secfion where fluxes are described (Line 151), the text should refer to the methods for how the calculafion was done: and since the rate of change of DO is based on the point samples data in figure 2c, the possibility that rates could be underesfimated should be noted.

Nature Communications manuscript NCOMMS-23-40695-T: Response to reviewers.
We would like to thank the reviewers for their very helpful and positive comments.We hope we have been able to modify the manuscript to their satisfaction.Below we provide a line by line response to the reviewers comments together with details of the resulting modifications to the paper.Our response to the reviewers is highlighted for clarity.

General comments:
In this manuscript, a mixing mechanism in the water column if seasonally stratified shelf seas is proposed that leads to additional reduction of oxygen saturation in the bottom waters during summer stratification.The peak of primary production in the thermocline (indicated by a strong subsurface chlorophyll maximum) leads to upward fluxes of oxygen above the maximum, including outgassing to the atmosphere.This reduces the potential for downward oxygen fluxes towards the bottom and increases there the oxygen deficit due to mineralisation of organic matter.This proposed mechanism is novel and relevant, and therefore the manuscript should be published in this journal.Some minor revisions would be necessary before.
As discussed below in more detail below, the argumentation could be formulated clearer, in wording as well as in improved tables and graphics.After reading the manuscript in depth, it is still not clear to me, if the authors simply claim that the bottom water oxygen saturation is further reduced simply due to outgassing of oxygen or if there are feedback mechanisms that further reduce the bottom oxygen concentration.
We are providing evidence to show that, due to the outgassing of some of the O2 generated by the SCM primary productivity, the O2 demands of the respiration and remineralisation associated with the sinking organic matter produced in the SCM will be greater than O2 mixed down.Hence adding to the deep water O2 decline.However, it's not a feedback mechanism.Hopefully the modification described below will clarify this further.

Detailed comments:
Abstract: It would be import to have clearer formulations of the impacts of the degassing of O2 into the atmosphere, like in lines 74-75.
The middle sentences (lines 22-25) have been rewritten based on the reviewers feedback.64-65: I do not fully understand this: "Accordingly, over the summer, diapcynal mixing promotes high carbon turnover in the deep water …" Would be nice to explain that by a few additional words.Does the carbon turnover profit from extra supply of oxygen?If so, this should be made clear.These sentences have now been re-written (lines 64-67) for clarity.74-75: Here the leakage as a cause for the O2 deficit is clearly formulated.Would be important to also have such a clear formulation in the abstract.

See above
Fig. 1: Would be good to mark the 17-deg C isotherm as a line.
We have actually added the 16-deg C isotherm, as this provides a clear demarcation between seasonally stratified and mixed regimes in the Celtic Sea which is what we think the reviewer was looking for?95-112: I find this paragraph very hard to read, because it reports several results from dissipation obervations at different time and different depths.This feels very descriptive.The authors should find a way to reformulate this paragraph, to give a clear message.
We have rewritten this paragraph as requested (lines 95-110) 110-112: "… This data quality issues …", not clear what these "issues" are, since observing dissipation rate inside the SML should not be an issue.
Yes -the reviewer is correct.This issue is not relevant to the narrative and so has been removed.115-116: Maybe better "an average downward diapycnal heat flux of 40 W/m2"?
We agree -this sentence has been rewritten accordingly.(line 114) 138-140: Wouldn't it be possible to estimate the oxygen flux to the atmosphere using bulk formulae as they are used in numerical models, depending on the wind speed and (maybe even the waves state)?
Yes -we have now done this (lines: ) with a description of the methodology added in the methodology section (lines: ).146: "there" instead of "There".done Figs.3c&h: Here the symbols should get a legend within the panel, for better readability.done 155-158: I guess these are the values for June, but you did not write it explicitly.
Yes -this is now stated.(line 253) 155-176: Although plausible, these numbers are not easy to digest.Would it be possible to additionally indicate these fluxes graphically, by some box plots with vertical arrows with size proportional to the fluxes?I know that space is limited in this journal, but such a graphics would help the reader tremendiously.Maybe, table 1 (which could be much better organised) could be replaced by such a graphics.
We have now added a schematic (figure 5) which shows the fluxes.To help the reader further we have coloured the flux estimates in table 1 the same colour as the arrows in figure 5.As there are two separate flux estimates we felt it would be clearer to retain the table and colour code in this way rather than trying to include both sets of flux estimates on the schematic.190: Why do you define N2/S2 = 1 as the value of marginal stability?Theory says that it should be 0.25.When you decrease N2/S2 from 1.0 to 0.9, the water column should still be stably stratified.This seems to contradict what you write ("any shear enhancement will potentially generate shear instability and mixing").
Okay -we have rewritten for clarity.See lines 188-194.The concept of marginal stability and generation of mixing by significant shear enhancement has previously been discussed in the references cited in this section.Fig. 4a: It would be more consistent to plot that wind contribution as wind speed times wind stress.It would also be better to apply a 25-hour filter on the U3 values.Then, in both cases, the pink and blue areas would be more plausible.
Yes -plot 4(a) did actually have the wind contribution as wind speed times wind stress, however, the figure legend in the submitted draft was incorrect for which we apologies.As requested by the reviewer we have now plotted the tidal contribution having applied a 25-hour filter as requested by the reviewer.
226-230: Here the argumentation could be more clear.I understand that the decay of oxygen in the deep water due to usage for mineralisation is larger than the supply from diapycnal mixing.Therefore, the oxygen concentration int he deep water is going down.But in what sense does this acclerate the removal?Are you saying that we have a self-enhancing process here?How does that work?
We have rewritten the discussion to improve clarity.I think the key point in response to the reviewer, is that the mixing process is mixing less O2 down from the SCM than is required to respire/ remineralise the sinking organic matter associated with the primary production in the SCM (the magnitude of which is determined by the simultaneous mixing up of nutrients).
Hans Burchard, 15.09.2023Reviewer #2 (Remarks to the Author): This is an excellent paper.Tt's very clear and straightforward despite the fact that it summarizes an extensive and diverse interdisciplinary data set.The main results are the clear demonstration that the mixing of DO downward from the pycnocline delays the decline in DO in the deepwater caused by respiration, and than the mixing flux is driven by interior shear driven by tides and augmented by near inertial oscillations.This paper should be read by all coastal marine scientists clarify the idea that low DO is due to reduced mixing because of summer increases in stratification.

Fig. 1 :
Fig. 1: Would be good to mark the 17-deg C isotherm as a line.